Is there still the material override feature within render layers in Blender 2.8?
The 2.8 version is not released yet. Even the developers cannot be absolutely sure about how everything is going to be when it's finished. Ability to override materials per render layer is an essential feature. Even though nothing can be said with absolute certainty at this moment it would be very logical to assume this functionality or a way to achieve similar results will remain in one way or another once Blender 2.8 is released. Once again, let's remember the obvious one more time: at this moment Blender 2.8 is not meant to be used for production and is expected to have a lot of serious issues.